Method for purification of an exhaust gas from a diesel engine

ABSTRACT

A method for removing impurities in exhaust gas from a diesel engine, where the impurities comprise nitrogen oxides, carbon monoxide, particulate matter and incompletely combusted hydrocarbons. The method comprises the steps of injection of a reductant comprising urea or ammonia into the exhaust gas from the engine, selective catalytic reduction of the nitrogen oxides in the exhaust gas by the reductant, and intermittent injection of a hydrocarbon into this effluent. The succeeding steps are oxidation of carbon monoxide, particulate matter, incompletely combusted hydrocarbons and injected hydrocarbon to carbon dioxide and water, and in selectively oxidising possible excess of reductant to free nitrogen, and finally filtration of the effluent by passing the gas through a catalysed filter. The remaining particulate matter is retained in the filter, and the carbon monoxide, particulate matter and hydrocarbons are oxidixed to carbon dioxide and water, and the reductant is selectively oxidixed to nitrogen, creating a purified exhaust gas.

The invention relates to a method for purification of an exhaust gasfrom an internal combustion engine.

The invention is specifically directed to cleaning of an exhaust gasfrom a diesel engine, especially engines in vehicles, which often startwith cold engine and cold exhaust gas system.

Processes for purifying exhaust gas are already known. In the process ofUS 2007/0289289 exhaust gas is purified by catching particles in afilter followed by reduction of nitrogen oxides and thereafter bycatalytic oxidation of impurities in the exhaust gas. However, if somenitrogen oxides pass the reducing catalyst, they are oxidised in thesubsequent step and escape to the atmosphere as NO₂. Further, if excessof a reductant is added, some of it may pass the oxidation catalyst andescape to the atmosphere.

In the process of US 2007/0160508 fuel is injected upstream of apre-stage oxidation catalyst, in which NO₂ is formed. NO₂ is used in thesubsequent filter for oxidising soot particles. Remaining NO_(x) isreduced by a reductant in the subsequent selective reducing catalyst,before the exhaust gas passes a post-stage oxidation catalyst, whichconverts CO to CO₂. In this process four different catalysts are needed.

Gas is led from a combustion engine to a diesel particulate filter inthe process disclosed in US 2007/0089403, where the filter is coatedwith an oxidation/NO_(x) storage catalyst. After injection of areductant the gas passes a hydrolysis catalyst before entering aselective reduction catalyst, after which it passes an ammonia guardcatalyst and streams to the atmosphere. This process is a littlecomplicated, the gas to the selective reduction is not sufficiently hotand must pass a hydrolysis catalyst before entering the reductioncatalyst, after which an ammonia guard catalyst is needed.

US 2006/0107649 discloses an exhaust gas cleaning process, where NO_(x)is reduced, particles are caught in a filter and CO, hydrocarbons andNO_(x) are thereafter oxidised. However, no additional heat can be addedto the filter to burn off caught soot and particulate matter.

The process of U.S. Pat. No. 6,892,529 comprises hydrogen injection,catalytic oxidation, hydrogen injection, removing particles in a filter,hydrogen injection, urea injection, hydrolysis, selective reduction ofNO_(x) and oxidation of CO and remaining hydrocarbons. A disadvantage ofthis process is that hydrogen is not easy to handle on board a vehicle.

Exhaust gas is cleaned in two parallel trains in the process disclosedin U.S. Pat. No. 6,823,660. In each train the gas passes an oxidationcatalyst, a diesel particulate filter and a selectively reducingcatalyst. This process does not supply sufficient heat for regenerationof the filter and control of NO_(x) reduction.

Reduction and subsequent oxidation of impurities in exhaust gas arecomprised in the process of U.S. Pat. No. 5,431,893. Reductant isinjected to the exhaust gas upstream of a pyrolysation channel, a mixingchannel and a reduction catalyst. The temperature of the oxidationcatalyst cannot be adjusted, and particles may block the filter or passthrough the filter.

The problem of the known processes is that they are either rathercomplicated or do not provide a thorough removal of both NO_(x), CO,remains of hydrocarbons, particulate matter and soot, especially notjust after start of a cold engine.

The problem of known technique is solved by the present invention.

The invention provides a method for removing impurities in exhaust gasfrom a diesel engine, where the impurities comprise nitrogen oxides,carbon monoxide, particulate matter and incompletely combustedhydrocarbons. The method comprises the steps of injection in excesscompared to stoichiometric ratio of a reductant comprising urea orammonia into the exhaust gas from the engine, selective catalyticreduction of the nitrogen oxides in the exhaust gas by the reductant inthe presence of a catalyst active in selective reduction of nitrogenoxides to nitrogen and intermittent injection of a hydrocarbon into thiseffluent. The succeeding steps are oxidation of carbon monoxide,particulate matter, incompletely combusted hydrocarbons and injectedhydrocarbon in the presence of a catalyst active in oxidising carbonmonoxide, particulate matter and hydrocarbons, to carbon dioxide andwater, and in selectively oxidising possible excess of reductant to freenitrogen, and filtration of the effluent by passing the gas through acatalysed filter, wherein the remaining particulate matter is retainedin the filter, and wherein the catalyst is active in oxidising carbonmonoxide, particulate matter and hydrocarbons to carbon dioxide andwater, and in selectively oxidising reductant to nitrogen creating apurified exhaust gas.

In another embodiment of the invention the method further comprises thestep of a pre-oxidation of carbon monoxide, particulate matter, nitrogenoxides and incompletely combusted hydrocarbons in the exhaust gas fromthe engine in the presence of a catalyst active in oxidising the carbonmonoxide, nitrogen oxides, hydrocarbons and particulate matter to carbondioxide, nitrogen dioxide and water prior to the injection of reductant.

Thereby exhaust gas from a diesel engine is very thoroughly cleaned by avery simple system, also reasonably fast after start of a cold engine.

FIG. 1 is a schematic drawing showing one preferred embodiment of themethod of the invention.

FIG. 2 is a schematic drawing showing another preferred embodiment ofthe method of the invention.

FIG. 3 is a print-out from measurement of pressure drop across dieselparticle filter during test.

Diesel engines operate with excess air and their exhaust gasses comprisenitrogen oxides, NO_(x), carbon monoxide, CO, particulate matter andincompletely combusted hydrocarbons, which all implement health risk.

The present invention provides a method, wherein nitrogen oxides arecatalytically, selectively reduced to free nitrogen. Subsequently CO andincompletely combusted hydrocarbons are oxidised. Finally, particulatematter is caught and remaining CO and incompletely combustedhydrocarbons are oxidised in a filter. These reactions take place in anoptimal way, when the exhaust gas and the system are heated up to250-500° C.

Optionally, the exhaust gas from the diesel engine is passed to apre-oxidising catalyst, where a considerable part of carbon monoxide,unburned hydrocarbons, particulate matter, and NO_(x) is oxidised tocarbon dioxide, water and NO₂ upstream of the selective, catalyticreduction of NO_(x).

In this case the reduction catalyst can be a zeolites catalyst.

The invention is described in more detail by the drawings. FIG. 1illustrates one preferred embodiment of the invention, where fuel 1 iscombusted with air 2 in combustion engine 3, and the formed exhaust gas4 is mixed with injected reductant 7. A preferred reductant is anaqueous solution of urea, which disintegrates to ammonia and carbondioxide at and above 200° C. This mixed gas flows to a Selective,Catalytic Reduction, SCR, catalyst 8, which promotes reduction ofnitrogen oxides by the reductant, ammonia, resulting in free nitrogenand water.

The catalyst for selective reduction can be a mixture of base metaloxides as the active phase supported on a carrier of one or more metaloxides. The base metals are chosen from vanadium, tungsten, cerium andmanganese, and the preferred catalysts are vanadium and tungsten oxidesupported on titania or alumina or ceria, or cerium oxide/tungsten oxidesupported on titania or alumina, or manganese oxide supported on titaniaor alumina or ceria. Alternatively, the catalyst for selective reductioncan be a zeolite, especially an ion exchanged zeolite supported on aninert substrate, preferably cordierite, and the preferred zeolites iscopper and/or iron exchanged beta or ZSM-5 zeolite. The catalyst willtypically be in the form of a monolithic structure but can also be inthe form of foam or metal mesh.

The reductant might also be ammonia or an aqueous solution of ammonia.The reductant can be added in a slight excess compared to stoichiometricratio, which ensures a very high degree of conversion of the poisonousnitrogen oxides to free nitrogen.

Hydrocarbon 10 is intermittently injected into SCR effluent 9, whenneeded for increasing the temperature. The hydrocarbon can be dieselfuel. This exhaust gas flows to a Diesel Oxidation Catalyst, DOC, 11where a substantial part of the CO and incompletely combustedhydrocarbons and particulate matter are oxidised to water and carbondioxide. Excess of ammonia is selectively oxidised to free nitrogen. Inthis way, the DOC 11 also acts as an ammonia slip guard.

The oxidation catalyst is a precious metal(s) catalyst on metal oxidecarriers such as aluminium oxide, cerium oxide, zirconium oxide titaniumoxide or a zeolite. The requirement to the amount of the noble metal islow. Precious metals are platinum, palladium or rhodium, which arepresent as mixtures or as single precious elements, where platinum andpalladium are the preferred metals, preferably on a titania support.

The precious metals can also be substituted by base metals, typicallymanganese, copper, cobalt and chromium.

The DOC catalyst will typically be in the form of a monolithicstructure, but can also be in the form of foam or metal mesh.

Both the amount of reductant and of hydrocarbon is monitored by theelectronic computing unit. This can both be a separate CPU or the CPU ofthe engine.

The nearly purified exhaust gas 12 flows to a catalysed DieselParticulate Filter, c-DPF, 13. Particulate matter is caught in thefilter and the catalyst on the surface of the filter promotes theoxidation of the particles as well as the selective oxidation ofremaining ammonia, carbon monoxide and hydrocarbons.

The catalyst is a Pt-free coat on the filter, which can be a cordieritefilter. The coat is a metal oxide acting as a carrier for a preciousmetal different from platinum, where the preferred precious metal ispalladium. The carrier coat is an oxide of cerium, zirconium, aluminiumor titanium, where the preferred oxide is titania.

The amount of hydrocarbons 10 injected up-stream of DOC 11 willinfluence the temperature not only in the DOC 11, but also in the c-DPF13 as well and thereby enhance combustion of collected particles by theincreased temperature of c-DPF.

By the exhaust gas cleaning process of the invention, the catalyst 8 forSCR is readily heated by warm exhaust gas coming directly from theengine to the temperature, where urea is disintegrated to ammonia andwhere nitrogen oxides are reduced.

Further, a very high degree of removal of nitrogen oxides can beobtained, as it is possible to inject excess of urea/ammonia reductant,and without high requirement of accuracy of injected amount, becauseslip of ammonia is oxidised to nitrogen not only by the DOC 11, but alsoby the c-DPF 13.

Thereby, the content of impurities in the purified exhaust gas stream 14is extremely low, when it leaves the system. FIG. 2 shows anotherpreferred embodiment of the invention. Fuel 1 is combusted by air 2 indiesel engine 3 and the formed exhaust gas 4 flows to a pre-oxidisingcatalyst 5, where a substantial part of CO, NO, particulate matter andremaining HC are oxidised, before this pre-oxidised exhaust gas 6 ismixed with reductant 7 and flows to SCR catalyst 8, where nitrogenoxides are reduced to free nitrogen. The NO_(x) free exhaust gas 9 isfurther cleaned in the same way as in the process described by FIG. 1.With this embodiment it is obtained that the formed nitrogen dioxidesupports the selective, catalytic reduction of nitrogen oxides to freenitrogen and that a zeolite catalyst can be used for the SCR reaction.

The pre-oxidising catalyst 5 consists of precious metals on one or moremetal oxide carriers as aluminium oxide, cerium oxide, zirconium oxidetitanium oxide or a zeolite where the requirement to the amount of thenoble metal is low. This catalyst shall have the ability to oxidise NOto NO₂ besides the ability to oxidise carbon monoxide and hydrocarbonsto carbon dioxide and water. A preferred catalyst is a mixture of theprecious metals platinum and palladium on aluminium oxide/cerium oxidecarrier. The precious metals can be mixtures or single preciouselements. The precious metals may be substituted by base metalsincluding manganese, copper, cobalt and chromium.

The pre-DOC catalyst will typically be in the form of a monolithicstructure but can also be in the form of foam or metal mesh.

The method of the invention is useful for systems for purifying exhaustgas from diesel engines, especially engines installed in cars, vans,vehicles, trains, vessels and power plants.

The performance of a system consisting of SCR+DOC+c-DPF catalysts wasevaluated in an Engine test bench on a Scania 12-1 Euro II enginethrough European transient test cycles, ETC.

The amount of injected urea solution was varied as shown in Table 2,whereas no hydrocarbon was injected upstream of the DOC.

Finally, the exhaust gas was passed through catalysed DPF.

The catalysts used for the evaluation were from current Haldor TopsoeA/S development:

-   -   DNXV standard SCR catalyst—vanadium based    -   Hi-DOC—Pt/TiO₂ oxidation catalyst    -   BMC-211 coated cordierite DPF

Further specifications of SCR, DOC and DPF are given in Table 1.

TABLE 1 Catalyst Volume to cylinder Size (litres) ratio Composition DNXVø12.7″ × 295 mm 24 2 V₂O₅/WO₃ on SCR TiO₂ DOC ø10.5″ × 100 mm 5.6 0.46Pt/TiO₂ DPF ø10.5″ × 12″ 17 1.42 BMC-211: Pt-free coat on cordieritefilter

The measured concentrations of impurities and the temperature in thestreams are given in Table 2.

TABLE 2 Urea CO No_(x) HC inj CO NO_(x) HC Max NH₃ conv conv conv [g][g/kWh] [g/kWh] [g/kWh] [ppm] [%] [%] [%] Upstream SCR 1  0 3.26 10.143.88 0.00 Upstream SCR 2  0 3.82 10.17 4.19 0.00 Downstream SCR  0 3.1610.85 0.02 0.00 3 −7 99 Downstream DOC  0 0.20 10.58 0.03 0.00 94 −4 99Downstream  0 0.14 9.88 0.00 0.00 96 3 100 Filter Downstream SCR 5863.40 4.09 0.02 9.56 −4 60 100 Downstream SCR 809 3.33 1.85 0.02 17.96 −282 100 Downstream SCR 929 3.41 0.98 0.03 159.00 −5 90 99 Downstream SCR1 033   3.17 0.52 0.03 392.06 3 95 99 Downstream SCR 1 169   3.39 0.570.03 473.00 −4 94 99 Downstream SCR 1 307   3.27 0.40 0.03 643.00 0 9699 Downstream DOC 815 0.21 1.82 −0.01 3.29 93 82 100 Downstream 813 0.151.71 −0.01 3.44 96 83 100 Filter Downstream DOC 925 0.18 1.26 0.01 3.0995 88 100 Downstream 902 0.40 1.31 0.01 7.32 88 87 100 Filter DownstreamDOC 1 041   0.19 1.29 0.01 3.86 94 87 100 Downstream 1 042   0.17 1.240.07 4.39 95 88 98 Filter

The efficiency of removal of particulate matter was determined asbuild-up of pressure drop across the filter and given in FIG. 3.

The allowed values for emissions from trucks in Europe are given inTable 3.

TABLE 3 European legislation based on the European Transient Cycle(ETC). Euro VI is a proposal of 2007.12.21. All units are in g/kWh TierDate Test CO NMHC CH4 NO_(x) PM Euro III 2000.10 ETC 5.45 0.78 1.6 5.00.16 Euro IV 2005.10 4.0 0.55 1.1 3.5 0.03 Euro V 2008.10 4.0 0.55 1.12.0 0.03 Euro VI 2013.04 4.0 0.16 0.5 0.4 0.01

The test results show clearly that particulate matter is oxidised, as itcan be seen in FIG. 3 that the pressure drop across the c-DPF does notincrease during operation, as shown.

From Table 2 it is seen that the HC, CO and NOx conversions areexcellent and that the NO_(x) emission in the outlet exhaust stream 14is very low. The main reason for the high NOx conversion is the NH₃/NOxratio can be maintained at a high value as the potential NH₃ slip isselectively oxidised both on the DOC 11 and on the catalytic coatedfilter 13.

A comparison between the limits of present legislation in Table 3 andtest results shows:

The obtained 1.24-1.71 g/kWh NO_(x) is lower than 2.0 (2008)

The obtained 0.15-0.40 g/kWh CO is lower than 4.0 (2008)

The obtained 0-0.07 g/kWh HC is lower than 1.1 (2008).

The above clearly demonstrates that the legislation is fulfilled.

The engine, which was used for the tests, was an old engine and theemission of impurities was much higher that the emission from modernengines. The purification system of the invention will easily fulfilfuture Euro VI requirements for limits of emissions from modernvehicles.

1. A method for removing impurities in exhaust gas from a diesel engine,where the impurities comprise nitrogen oxides, carbon monoxide,particulate matter and incompletely combusted hydrocarbons, which methodcomprises the steps of (a) injection in excess compared tostoichiometric ratio of a reductant comprising urea or ammonia into theexhaust gas from the engine; (b) reduction of the nitrogen oxides in theexhaust gas by the reductant in the presence of a catalyst active inselective reduction of nitrogen oxides to nitrogen; (c) intermittentinjection of a hydrocarbon into the effluent from step (b); (d)oxidation of carbon monoxide, particulate matter, incompletely combustedhydrocarbons and injected hydrocarbon in the presence of a catalystactive in oxidising carbon monoxide, particulate matter andhydrocarbons, to carbon dioxide and water, and in selectively oxidisingexcess of reductant to free nitrogen; (e) filtration of the effluentfrom step (d) by passing the gas through a catalysed filter, wherein theremaining particulate matter is retained in the filter, and wherein thecatalyst is active in oxidising carbon monoxide, particulate matter andhydrocarbons to carbon dioxide and water, and in selective oxidisingreductant to nitrogen, creating a purified exhaust gas; and (f)withdrawal of the purified exhaust gas.
 2. A method according to claim1, further comprising the step of a pre-oxidation of carbon monoxide,particulate matter, nitrogen oxides and incompletely combustedhydrocarbons in the exhaust gas from the engine in the presence of acatalyst active in oxidising the carbon monoxide, nitrogen oxides,hydrocarbons, and particulate matter to carbon dioxide, nitrogen dioxideand water prior to the step (a).
 3. A method according to claim 1,wherein the injected hydrocarbon is diesel fuel.
 4. A method accordingto claim 1, wherein the catalyst for selective reduction is a zeolite oran ion exchanged zeolite on a cordierite catalyst support, or one ormore base metal oxides catalyst on a catalyst support of one or moremetal oxides, the catalyst having the form of a monolite, a foam or ametal mesh.
 5. A method according to claim 1, wherein the oxidationcatalyst is one or more precious metals or one or more base metals on acatalyst support of a zeolite or a metal oxide, the catalyst having theform of a monolite, a foam or a metal mesh.
 6. A method according toclaim 1, wherein the catalyst coated on the filter is a precious metal,but not including platinum, and on a catalyst support of a metal oxide.7. A method according to claim 2, wherein the preoxidation catalyst isone or more precious metals or one or more base metals on a catalystsupport of a zeolite or of an oxide of one or more metals, the catalysthaving the form of a monolite, a foam or a metal mesh.
 8. A methodaccording to claim 1, wherein the catalyst base metal is one or more ofvanadium, tungsten, cerium and manganese, the support metal oxide istitania, alumina and/or ceria and the ion exchanged zeolite is Cu/Feexchanged β zeolite or ZSM-5 zeolite.
 9. A method according to claim 1,wherein the support for the oxidation catalyst is a zeolite, titania,alumina, ceria or zirconia, and wherein the catalyst precious metal isplatinum, palladium and/or rhodium and the catalyst base metal ismanganese, copper, cobalt and/or chromium.
 10. A method according toclaim 1, wherein the support for the catalyst is titania, alumina, ceriaor zirconia, and the catalyst is palladium.
 11. A method according toclaim 2, wherein the catalyst support is one or more oxides ofaluminium, cerium, zirconium and/or titanium, and the catalyst preciousmetal is platinum and/or palladium and the catalyst base metal ismanganese, copper, cobalt and/or chromium.
 12. A method according toclaim 1, wherein the catalyst is vanadium/tungsten oxide on a titaniasupport, cerium/tungsten oxide on a titania support or manganese oxideon a titania support.
 13. A method according to claim 1, wherein thecatalyst is platinum on a titania support or platinum/palladium on atitania support.
 14. A method according to claim 1, wherein the catalystis palladium on a titania support.
 15. A method according to claim 2,wherein the catalyst is platinum/palladium on an aluminium/cerium oxidesupport.